When hydrogen is used as a fuel in motor vehicles, a hydrogen fuel depot infrastructure for refueling must also be developed. Typically, in the use of hydrogen to power fuel cells or in the use of CNG to power internal combustion engines in motor vehicles, present practice is that high pressure fuel gas is stored in on board fuel tanks maintained at a maximum design pressure in the range of about 3600 psi for CNG to about 5000 psi for hydrogen. Achieving a full refill of gas in the tank to design specification and an increase in the energy efficiency of the overall system of refuel depots and on board vehicle tanks and their interrelationship are desirable goals.
Hydrogen powered vehicles utilize light weight polymer/composite hydrogen storage tanks to store hydrogen fuel on board at high pressure. Herein, reference to hydrogen powered vehicles correlates with the use of the invention with compressed natural gas powered vehicles (CNGVs). For clarity, hydrogen is referred to in the specification and is a term intended to be interchangeable, generally evident in context, with compressed natural gas, high pressure gas, or gas. The use of multiple cylindrically shaped small tanks rather than a single large tank is preferred for vehicle design purposes. Various designs for high pressure hydrogen refueling systems have been proposed. When the storage tank of a hydrogen powered vehicle is filled with hydrogen, the pressurized on board storage tanks for the gas may be characterized as including chemical energy from the gas itself (consumed in powering the vehicle), and mechanical and thermal energy associated with the high pressure under which the gas is stored at the refuel depot and refueled into the vehicle tank[s].
During a high pressure refueling process with hydrogen or CNG, the interior of the on board tanks, namely, the gas itself, becomes heated as a result of gas compression as the tank pressure increases and other refueling parameters affect the process. After refueling, the tank interior gas temperature and pressure decrease slowly as the fuel gas is consumed during vehicle operation. Conventionally, it is not usually possible to obtain a full refill tank pressure to a high pressure design maximum without some form of secondary gas processing. In one example of pressure compensation during the course of refueling, the charge of fuel input into and stored in the tank must be initially in excess of the optimum design tank pressure because of the compression/heating effect: as temperature increases, less fuel per unit of tank volume can be accepted by the tank. In another example of secondary treatment, the refuel gas is precooled before input into the tank. Without a full charge of fuel, vehicle mileage in terms of vehicle range is reduced; the use of higher design pressures worsens this condition. A third variation of an attempt to resolve the less than full fill problem involves a slow flow rate during refill resulting in a lower initial tank temperature, however, a slow fill, is undesirable, and may be impractical. An undesirable consequence of a slower flow rate during refueling to avoid heat build up is a longer refueling time. Solutions proposed to precool the gas before refueling and to initially overfill require substantial energy, thereby reducing the overall energy efficiency of a hydrogen economy. The build up of the compression heat of refueling is generally not a concern when fill pressures are at about 3600 psi and 5000 psi or lower, however, as refill and tank pressures exceed 3600 psi and 5000 psi and approach or exceed 5000 psi and 10,000 psi, temperature compensation, cooling, becomes an important factor in the refueling process to achieve a full fill. With a full fill, overall vehicle range per each tank refill is extended and overall customer satisfaction is increased.